One fun fact about sand dollars is that these animals are closely related to sea urchins. In addition, the white or off-white sand dollar skeletons found on beaches are called "tests," and their five-jawed mouths are at the center of the star-shaped markings on the tests.
Sand dollars are echinoderms, like sea urchins, sea cucumbers and starfish. Live sand dollars are covered in short spines that come in a variety of colors, including purple, green, black and reddish brown. They use these spines to burrow into the sand, move along the sand and collect food particles. When the particles come to rest on the spines, the animal uses its pincers, cilia and five tube feet to transport the food to its mouth.
The mouth, called Aristotle's lantern, consists of five jaw sections used to grind up plant and animal matter. A sand dollar may spend as long as 15 minutes grinding its food before swallowing it, and it can take as long as two days to digest its food.
The shell-like tests found washed up on beaches are sand dollar endoskeletons. The tests consist of rigid, fused plates and differentiate sand dollars from other echinoderms, which have skeletons made up of smaller, more flexible skeletal plates or small bony structures called ossicles.